The present invention relates to a catalyst for use in the hydrocracking of a heavy hydrocarbon feedstock so as to increase cracking activity and obtain high quality middle distillate products boiling in the range of about between 300.degree. F. to 700.degree. F. and a process for the treatment of a heavy hydrocarbon feedstock with the catalyst.
In the petroleum industry there is a great interest in converting conventional HDS plants into mild hydrocracking plants. This interest is mainly due to the increase in demand of high quality middle distillate products boiling in about the range of 300.degree. F. to 700.degree. F. These high quality middle distillate products include, for example, aviation turbine fuels, diesel fuels, heating oils, solvents and the like. In order to meet the growing demand for high quality middle distillate products it is desirable to complement the distillation and catalytic cracking operations with catalytic hydrocracking.
Heretofore, most investigations on catalytic hydrocracking have been oriented towards production of lower boiling point products such as gasoline. In known processes it has been necessary to prepare very active catalysts which are normally obtained from a crystalline hydrogen Y zeolite base or silica-alumina base upon which is deposited suitable hydrogenation metal components. The type of catalyst as mentioned above cannot readily be used for the conversion of heavy gas oils boiling above 700.degree. F. for middle distillate production because of significant changes in selectivity. The foregoing catalysts are generally used under severe conditions and under these conditions with a heavy feedstock a large portion of the feed is converted to products boiling below 400.degree. F.
Attempts to use conventional HDS cobalt-molybdate catalysts, which are known to have some moderate cracking activity, under hydrocracking conditions to obtain a more selective conversion of heavy feeds to middle distillate products, improved selectivity and cracking activity have been unsuccessful. For example, U.S. Pat. No. 3,853,742 discloses the addition of small amounts of zeolite to a hydrorefining catalyst of the type set forth above to improve activity. While the addition of zeolite improved catalyst activity, selectivity of the catalyst was sacrificed. U.S. Pat. No. 3,306,843 discloses incorporating silica gel into a catalyst in order to improve cracking activity. Again, the cracking activity improved at the expense of selectivity of the catalyst which was drastically reduced. Other examples of mild hydrocracking catalysts of the type known in the prior art are U.S. Pat. Nos. 4,396,500, 3,755,150, 3,287,280 and 3,232,887 wherein an active phase of nickel, molybdenum and phosphorus is impregnated on a gamma alumina support. Finally, hydrocracking catalysts based on zeolites are disclosed in U.S. Pat. Nos. 3,897,327, 3,506,400, 3,513,108, 3,354,077, 3,449,070 and 3,293,192. While the foregoing prior art catalysts offer some improvement in cracking activity, they do not meet the overall criteria of increasing cracking activity so as to obtain high quality middle distillate products boiling in the range of 300.degree. F. to 700.degree. F.
Accordingly, it is a principal object of the present invention to provide an improved catalyst for the mild hydrocracking of heavy vacuum gas oils.
It is a particular object of the present invention to provide an improved catalyst for use in the mild hydrocracking of heavy hydrocarbon feedstocks wherein the surface of the catalyst is characterized by a silica-alumina material having textural and crystallographic properties substantially similar to the alumina carrier.
It is a further object of the present invention to provide a catalyst as aforesaid wherein the bulk of the catalyst is substantially free of silicon.
It is a still further object of the present invention to provide a process for the treatment of heavy hydrocarbon feedstocks with a catalyst.
Further objects and advantages will appear hereinbelow.